<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <button>按钮</button>
    <script>
        var btn = document.querySelector('button');
        var onOff = true;
        btn.onclick = function(){
            if(onOff){
                alert('左边');
            }else{
                alert('右边')
            }
            onOff = !onOff;
        }
    </script>
</body>
</html>